The present invention relates to a digital image recording system which is used for security purpose, etc. and capable of recording images on occurrence of events. In particular, the present invention relates to a measure for providing a system which can avoid missing important image information, without necessarily using a high-speed image acquisition device.
As a conventional surveillance image recording system for security or like purpose, there has been widely utilized a surveillance video system which uses a magnetic tape or the like and records image signals from an image taking device such as a surveillance camera. However, the system of this type, which records analog signals on a magnetic tape, is troubled with deterioration of recorded images and complex maintenance involving tape exchange, etc. To solve such problems, recent attempts have been directed to development of a digital image recording system which applies the digital technology and utilizes a hard disk or the like as the recording medium (e.g. Japanese Patent Laid-open Application H6-113251 (JP-A-1994-113251)).
In the system of this type, image information on a surveillance area is recorded either intermittently at predetermined time intervals (intermittent recording mode) or only when an alarm is inputted by a sensor or the like (e.g. Japanese Patent Laid-open Application H10-290449 (JP-A-1998-290449)). Generally, the system of this type employs several cameras. In this case, the recording pattern of each camera is set depending on its built environment, so that image information on the surveillance area can be intermittently recorded in accordance with this setting. Otherwise, a recording mode is set on the image recording device in such a manner that an alarm signal from a sensor, etc. triggers image recording around the alarm signal transmission time, whereby images are recorded as specified by such setting.
Nevertheless, because this system of conventional type records an image simultaneously with the occurrence of an event, the speed of the image acquisition device governs the speed of the whole system. For example, a high-performance system can be constructed with an image acquisition device capable of getting 30 images per second. In contrast, with an image acquisition device capable of getting as little as about 10 images per second, the performance of this system is seriously degraded in comparison with that of the former. In other words, the latter system may fail to record image information on some of event occurrences, when 10 or more events take place in a second. Therefore, in an attempt to avoid missing image information, a conventional system has been constructed using a high-speed image acquisition device.
Nonetheless, where the recording mode is set for every camera as described earlier, valuable information in the surveillance area (e.g. presence of an intruder) may be missed during the intermittent recording mode operation. On the other hand, according to the recording mode where an image is recorded only on acquisition of an event, no image can be recorded before transmission of an alarm signal.
As a solution to these problems, use of a plurality of image recording systems has been suggested. But this measure is not practical, not only because of the necessity for a large installation space to accommodate the resultant huge system, but also because of the increase of the cost for the whole system.
From the viewpoint of reducing the cost for the system, it is effective to use a relatively low-speed image acquisition device which obtains about 10 images per second.
However, when the low-speed image acquisition device is mounted on the conventional system without any arrangement, the system fails to record the image information on some of event occurrences as mentioned above. The system may miss valuable image information depending on the situation, thus being unsatisfactory in terms of reliability.
Further, where the number of channels is increased to deal with multichannel applications, it is necessary to forbid an asynchronous input by each channel and to depend exclusively on a synchronous switching. Otherwise, to allow asynchronous inputs, the image acquisition devices are required as many as the channels.
Nevertheless, synchronous switching causes unnecessary switching of cameras, which tremendously increases the volume of recorded information. On the other hand, to allow asynchronous inputs, a large number of image acquisition devices are necessary, which inevitably raises the cost for the whole system.
The present invention has been worked out in view of these concerns. Its object is to provide an image recording system utilizing a relatively low-speed image acquisition device, in which the single image acquisition device is capable of recording image information from a plurality of cameras without fail.
In order to accomplish the above object, the present invention separates the input of an event and the acquisition of image information, and allows an event input operation and an image information acquisition operation to proceed asynchronously. As a consequence, even if the image information acquisition means operates at a relatively low speed, image information can be obtained without fail, as far as the speed of the event input means is fast enough relative to that of the event occurrence.
In this specification, it should be understood that xe2x80x9cimage informationxe2x80x9d not only indicates xe2x80x9cdata of an image itselfxe2x80x9d but also includes various incidental information (diverse information such as the importance of an obtained image).
A digital image recording system of the present invention is described with reference to FIG. 1. The image recording system comprises a plurality of image taking means 2, 2, . . . , recording means 4 which is capable of recording image information taken by the image taking means 2, 2, . . . , event storage means 5 for storing, on occurrence of events, the order of such occurrences, and image acquisition means 3 which imports the image information from the image taking means 2 in connection with the event, according to the order of event occurrences stored in the event storage means 5, and which records the image information on the recording means 4.
In this arrangement, when there happen a plurality of events to be regarded as image import requests, the event storage means 5 starts to store the order of event occurrences. Even if many events take place in a short period, the event storage means 5 stores the order of each occurrence, without missing important information. Based on and in compliance with the order of event occurrences stored in the storage means 5, the image acquisition means 3 imports image information from the image taking means 2 in connection with the event, and records the image information on the recording means 4. Therefore, even though the image acquisition means 3 imports images at a low speed, the image information on the events is recorded on the recording means 4 in the order of such occurrences, so that the image information can be imported without fail. Besides, the image acquisition means 3 need not be installed in plurality in correspondence with the number of the image taking means 2. Namely, irrespective of the speed of event occurrences, the single image acquisition means 3 is capable of recording image information without fail from a plurality of image taking means 2, 2, . . .
The image acquisition means 3 may be further capable of conducting multiprocessing of intermittent image information which is intermittently imported from the image taking means 2 and alarm image information which is transmitted, on generation of an alarm, from the image taking means in connection with such alarm generation.
With regard to the channel for importing image information from the single image taking means 2 into the image acquisition means 3, this arrangement does not require an independent channel for intermittent image information and another channel for alarm image information. In other words, the image acquisition means 3 can import both information via one channel only. The expression xe2x80x9con generation of an alarmxe2x80x9d as used herein indicates, for example, the moment when an alarm signal is transmitted by a sensor for detecting an intruder or when a human-operated alert switch is turned on.
Further, there may be provided importance level storage means 6 for recognizing and storing the level of importance of events, when the event storage means stores the order of event occurrences. Afterwards, when the image information on each event is recorded on the recording means 4, the importance level of each event can be added to the image information.
With this arrangement, the information indicating the importance level of the image information is recorded on the recording means 4. Hence, it is possible to retrieve and fetch only the information of particular value (the information with a high level of importance) among a volume of image information recorded on the recording means 4.
Furthermore, the event storage means 5 can be arranged to store, on occurrence of events and in the order of event occurrences, the channel number corresponding to the image taking means in connection with each event. Besides, provided that the channel number for the image taking means 2 in connection with one of such events is the same as the one already existing in the event storage means 5 at the time of that event occurrence, the channel number corresponding to the image taking means in connection with the late event is excluded from additional storage. In this case, the importance level storage means 6 compares the levels of importance of both events related to the same channel number. Only when the event to be stored shows a higher level of importance, the importance level of the event to be stored is stored in replacement of the importance level of the existing event.
Even when a plurality of events take place in a short period of time in connection with a certain channel number, this arrangement prohibits the event storage means from additionally storing the same channel number on every occurrence. Thus, it is possible to prevent disproportionate import of image information from a particular channel, so as not to affect image import timing of other channels. If image information is imported excessively from a certain channel, particularly when the image acquisition means 3 operates at a relatively low speed, the import of image information from other channels may be hampered. On the contrary, the present arrangement can prevent such a situation.